In this age of the growing importance of information, organic EL devices have been highlighted as displaying device of the next generation. Hitherto, cathode-ray tubes have been predominantly used in information displaying equipments in larger sizes. Cathode-ray tubes are however large in both volume and weight and high in operation voltage, thus they may be unsuitable in compact equipments.
More required are information displaying equipments which are in a thinner and lighter panel form and operable with a lower voltage and less power consumption. Liquid crystal devices have been extensively used in various fields because of the merit that they are operable with a lower voltage and less power consumption.
Liquid crystal devices however have the demerits that one can hardly receive a clear information therefrom when he or she views them at an angle outside the specific ranges, as well as that their power consumption is not so low as expected because they usually require backlight. Organic EL devices have been proposed as information displaying means which may overcome the above demerits.
Organic EL devices are classified into light-emitting devices which utilize a luminescence such as fluorescence or phosphorescence: They usually comprise a luminescent layer incorporated with a luminescent compound and inserted between a cathode and anode to which dc voltage is energized to inject holes and electrons in the luminescent layer so that a pair of hole and electron recouple each other to make in the luminescent compound an excited state which subsequently returns to the ground state to emit such a luminescence.
Organic EL devices are characterized in that their luminescent color tint can be controlled to a desired level by selecting an appropriate organic compound to be used as host compound in forming a luminescent layer, and screening a guest compound (or dopant) which may most suit to the host compound. Further, luminescence brightness and life expectancy may be remarkably improved, depending upon the combination of host-and guest-compounds.
Organic EL devices have been deemed to be excellent in principle because of the fact that they emit light in an autonomous manner, and this removes the dependency of visual field angle from information displaying equipments. Also, that they require no backlight and this would advantageously save power consumption.
Many of organic EL devices proposed hitherto however have a problem of durability: Their brightness may decay within a short period of time when used under severe conditions, for example, in case of equipping them to cars and automobiles where mechanical vibrations and high temperatures are unavoidable.
In display panels which are driven in dot matrix mode and required for elevated brightness, any brightness decay would be a serious problem. Such decay is most remarkable in green light-emitting devices which usually exhibit the highest brightness in full color panels. This may be due to the fact that there have been available no green dopant materials with a high efficiency and satisfactorily high thermal resistance.
As to green dopant materials, coumarin derivatives have been employed in usual cases. They may be useful as functional organic materials in the field of organic electronics, particularly, as luminescent materials in organic EL devices because they have absorption and luminescent maxima in the visible region, as well as having a property of emitting a visible light when excited.
3-(2-benzothiazolyl)-7-(diethylamino) coumarin, a typical high efficient green dopant material commercialized under the trade name of “COUMARIN 6” for use in conventional organic EL devices, has been proved to be insufficient for durability at elevated temperature because its glass transition point is not so high (99° C.). Because of this, it is hard to apply it to organic EL devices directed to use in cars and automobiles which causes elevated temperature, for example, 100° C. or higher.
In order to realize an enhanced durability which may render coumarin derivatives useful as dopant in organic EL devices, there have been proposed a series of coumarin derivatives which bear at the C-3 position in coumarin skeleton a benzothiazolyl group as substituent and form a julolidine ring including carbons at the C-6 to C-8 positions in coumarin skeleton, in addition to another series of coumarin derivatives where a hydrocarbon group is bound to the C-4 position in coumarin skeleton and a julolidine- and a benzothiazole-groups are placed at any positions other than the C-4 position in coumarin skeleton: Such a coumarin derivative may exhibit an enhanced durability at elevated temperature while retaining the high efficient green luminescence of COUMARIN 6 (see Japanese Patent Kokai Nos. 2001-76,876 and 2002-226,484).
Although coumarin derivatives as disclosed in Japanese Patent Kokai Nos. 2001-76,876 and 2002-226,484 have been proved to be capable of emitting a remarkable luminescence in the green region and this may render them very useful as luminescent agent in organic EL devices, as well as of much more enhancing durability at elevated temperature than COUMARIN 6, the improvement is not enough to apply them to organic EL devices directed to use in cars and automobiles where ambient temperature for organic EL devices becomes 100° C. or higher because the glass transition point of such a coumarin derivative is still 150° C. or lower.
In view of the above described problems, the objective of the present invention is to improve the durability at elevated temperature in organic EL devices where coumarin derivatives are used as dopant in luminescent layer.